#84 Better backup handling

$ ait --backup-dir /home/kev/.backups
# ait --backup-dir /home/root/.backups

While adding `ait` to [pkgsrc](https://pkgsrc.org) I noticed that the way the backup stuff is done is pretty bad when installed system-wide not via the source itself. This is because there is only 1 possible backup directory and no way to change it. I may add a way to change the location of the backup directory via a command line option such as: ``` $ ait --backup-dir /home/kev/.backups # ait --backup-dir /home/root/.backups ``` I really want to stay away from a config file and this is one way to avoid it. Plus, you could always make an alias if you don't want to do it all the time. Or maybe there can be a default in the source or something.